1. Field of the Invention
The present invention relates to a method, system, and program for performing error correction in a storage device having a magnetic storage medium.
2. Description of the Related Art
Computer hard disk drives include one or more disks of magnetic storage medium and a disk drive head assembly to read and write data on the magnetic storage medium. Magnoresistive (MR) heads typically include a write element comprised of a thin film inductive head and a read element comprised of a sensor. MR heads typically are affixed to an actuator or arm that glides across the disk surface to position the head at different track locations. The disk surface on which the data is written comprises a magnetic storage medium that is composed of magnetic grains. A magnetic grain comprises a collection of atoms which can be polarized as a unit in one direction. Each bit is comprised of a group of magnetic grains, where the bit indicates a value if a sufficient amount of the magnetic grains that comprise the bit are oriented in the direction of a discernible charge, i.e., a “0” or “1” orientation, where the MR head reads the charge.
Errors are introduced when the grains for a bit switch their orientation. If enough grains switch their orientation in response to the introduction of noise from various sources, such as heat or other magnetic fields, then the signal to noise (S/N) ratio for that bit decreases. If the noise increases to a certain point, then the intended orientation of the bit may not be determinable. One factor contributing to this drift in the orientation of the magnetic grains is thermal decay. The thermodynamics or heat of the disk system may cause grains to switch their orientation, thus increasing noise and possibly leading to disk errors.
In order to increase the linear density of disk drives, i.e., the tracks per inch, more bits and tracks must be packed onto the disk surface. One common technique for increasing disk density is to reduce the magnetic grain size to allow a greater number of grains and hence bits to fit on the magnetic recording surface. However, as the size and volume of the magnetic grains decrease, the grains become more susceptible to assuming a random value, i.e., switching their magnetic orientation, as a result of thermal decay thereby introducing more noise into the system.
Another factor introducing noise into the system is known as adjacent track interference (ATI). Adjacent track interference results from a write head, which may be writing on track, affecting the magnetic orientation of grains in adjacent bits, thereby introducing further noise into the system.
One prior art technique for remedying the noise introduced by thermal decay and ATI is to periodically reread and rewrite the data. Rewriting the data reorients the magnetic grains toward the intended charge, including any grains whose orientation switched as a result of thermal decay and misregistration. Prior art techniques will periodically perform this reread and rewrite operation to eliminate the introduced noise. To determine how often to perform the read and write operation, prior art techniques will design a thermal decay formula for a disk based on the characteristics of the disk to estimate the bit decay rate. The period for performing the reread and rewrite operation is sometimes set to a period that must pass before the amount of decay, i.e., change in the signal-to-noise ratio, reaches a threshold value based on the thermal decay equation.
Notwithstanding such prior art techniques for reversing the effects of thermal decay and other problems related to the addition of noise in the system, there is a need in the art for continued advancements in the area of reducing disk noise and degradation of the signal-to-noise ratio.